Left-hand traffic (LHT) and right-hand traffic (RHT) are the practice, in bidirectional traffic, of keeping to the left side or to the right side of the road, respectively. A fundamental element to traffic flow, it is sometimes referred to as the rule of the road.[1]

RHT is used in 165 countries and territories, with the remaining 75 countries and territories using LHT.[2] Countries that use LHT account for about a sixth of the world's area with about 35% of its population and a quarter of its roads.[3] In 1919, 104 of the world's territories were LHT and an equal number were RHT. From 1919 to 1986, 34 of the LHT territories switched to RHT.[4]

For rail transport, LHT predominates in Western Europe (except Germany, Denmark, Austria, Spain, and the Netherlands), Latin America (except Mexico), and in countries formerly in the British and French Empires, whereas North American and central and eastern European train services operate RHT.[citation needed]

In LHT vehicles keep left, and cars are RHD (right-hand drive) with the steering wheel on the right-hand side and the driver sitting on the offside or side closest to the centre of the road. The passenger sits on the nearside, closest to the kerb. Roundabouts circulate clockwise. In RHT everything is reversed: cars keep right, the driver sits on the left side of the car, and roundabouts circulate counterclockwise.

Traffic moves from left to right in Stockholm, Sweden, on 3 September 1967

Ancient Greek, Egyptian, and Roman troops kept to the left when marching.[6] In 1998, archaeologists found a well-preserved double track leading to a Roman quarry near Swindon, in southern England. The grooves in the road on the left side (viewed facing down the track away from the quarry) were much deeper than those on the right side, suggesting LHT, at least at this location, since carts would exit the quarry heavily loaded, and enter it empty.[7] In the year 1300, Pope Boniface VIII directed pilgrims to keep left.[6]

Sweden switched to RHT in 1967, having been LHT since from about 1734[11] despite having land borders with RHT countries, and approximately 90 percent of cars being left-hand drive (LHD) vehicles.[12] A referendum was held in 1955, with an overwhelming majority voting against a change to RHT. Nevertheless, some years later the government ordered a conversion, which took place at 5 am on Sunday, 3 September 1967. The accident rate dropped sharply after the change,[13] but soon rose back to near its original level.[14] The day was known as Högertrafikomläggningen or Dagen H for short. When Iceland switched the following year, it was known as Hægri dagurinn or H-dagurinn.[15] Most passenger cars in Iceland were already LHD.

In Italy the countryside was RHT while cities were LHT until 1927.[21] Rome changed to RHT in 1924 and Milan in 1926. Alfa Romeo and Lancia did produce RHD cars until as late as 1950 and 1953 respectively only to special order, as many drivers favoured the RHD layout even in RHT as this offered the driver a clearer view of the edge of the road in mountainous regions at a time when many such roads lacked barriers or walls.[22] The Rome Metro uses LHT.

LHT was introduced in British West Africa. All of the countries formerly part of this colony have borders with former French RHT jurisdictions and have switched to RHT since decolonization. These include Ghana, The Gambia,[25] Sierra Leone, and Nigeria. LHT was introduced by the British in the East Africa Protectorate (now Kenya, Tanzania and Uganda), Rhodesia, and the Cape Colony (now Zambia, Zimbabwe and South Africa). All of these have remained LHT. Sudan, formerly part of Anglo-Egyptian Sudan switched to RHT in 1973, as it is surrounded by neighbouring RHT countries.

Rwanda and Burundi, former Belgian colonies in Central Africa, are RHT but are considering switching to LHT[26][27] like neighbouring members of the East African Community (EAC).[28] A survey, carried out in 2009, indicated that 54% of Rwandans were in favour of the switch. Reasons cited were the perceived lower costs of RHD vehicles as opposed to LHD versions of the same model, easier maintenance and the political benefit of harmonisation of traffic regulations with other EAC countries. The same survey also indicated that RHD cars are 16 to 49 per cent cheaper than their LHD equivalents.[29] In 2014 an internal report from consultants to the Ministry of Infrastructure recommended a switch to LHT.[30] In 2015, the ban on RHD vehicles was lifted; RHD trucks from neighbouring countries cost $1000 less than LHD models imported from Europe.[31][32]

In Japan, Post-World War II Okinawa was ruled by the United States Civil Administration of the Ryukyu Islands and was RHT. It was returned to Japan in 1972 but did not convert back to LHT until 1978.[52] The conversion operation was known as 730 (Nana-San-Maru, which refers to the date of the changeover, 30 July). Okinawa is one of few places to have changed from RHT to LHT in the late 1900s.

Vietnam became RHT as part of French Indochina, as did Cambodia. In the latter country, RHD cars, many of which were smuggled from Thailand, were banned from 2001, even though they accounted for 80% of vehicles in the country.[53]

Samoa, a former German colony, had been RHT for more than a century. It switched to LHT in 2009,[54] being the first territory in almost 30 years to switch.[55] The move was legislated in 2008 to allow Samoans to use cheaper right-hand drive (RHD) vehicles—which are better suited for left-hand traffic—imported from Australia, New Zealand or Japan, and to harmonise with other South Pacific nations. A political party, The People's Party, was formed to try to protest against the change, a protest group which launched a legal challenge,[56] and an estimated 18,000 people attending demonstrations against it.[57] The motor industry was also opposed, as 14,000 of Samoa's 18,000 vehicles are designed for RHT and the government has refused to meet the cost of conversion.[55] After months of preparation, the switch from right to left happened in an atmosphere of national celebration. There were no reported incidents.[3] At 05:50 local time, Monday 7 September, a radio announcement halted traffic, and an announcement at 6:00 ordered traffic to switch to LHT.[54] The change coincided with more restrictive enforcement of speeding and seat-belt laws.[58] That day and the following day were declared public holidays, to reduce traffic.[59] The change included a three-day ban on alcohol sales, while police mounted dozens of checkpoints, warning drivers to drive slowly.[3]

Brazil was a colony of Portugal and switched to RHT with Portugal in 1928.[60] Other Central and South American countries that later switched from LHT to RHT include Argentina, Chile, Panama,[61] Paraguay,[62] and Uruguay.

In healthy populations, traffic safety is thought to be the same regardless of handedness, although some researchers have speculated that LHT may be safer for ageing populations[64] since humans more commonly have right-eye ocular dominance.[65][66] Comparing accident statistics between countries operating either LHT or RHT, Leeming concluded that LHT is superior.[67] However, Watson has criticised the small sample size and dismisses the notion.[4]

Although many LHT jurisdictions are on islands, there are cases where vehicles may be driven from LHT across a border into a RHT area. Such borders are mostly located in Africa and southern Asia. The Vienna Convention on Road Traffic regulates the use of foreign registered vehicles in the 74 countries that have ratified it.

LHT Thailand has three RHT neighbors: Cambodia, Laos, and Myanmar. Most of its borders use a simple traffic light to do the switch, but there are also interchanges which enable the switch while keeping up a continuous flow of traffic.[68]

There are four road border crossing points between Hong Kong and Mainland China. In 2006, the daily average number of vehicle trips recorded at Lok Ma Chau was 31,100.[69] The next largest is Man Kam To, where there is no changeover system and the border roads on the mainland side Wenjindu intersect as one-way streets with a main road.

The Takutu River Bridge (which links LHT Guyana and RHT Brazil[70]) is the only border in the Americas where traffic changes sides.

In RHT jurisdictions, vehicles are configured with LHD, with the driver sitting on the left side. In LHT jurisdictions, the reverse is true. The driver's side, the side closest to the centre of the road, is sometimes called the offside, while the passenger side, the side closest to the side of the road, is sometimes called the nearside.[71]

Most windshield wipers are designed to clear the driver's side better and have a longer blade on the driver's side[72] and wipe up from the passenger side to the driver's side. Thus on LHD configurations, they wipe up from right to left, viewed from inside the vehicle, and do the opposite on RHD vehicles.

Historically there was less consistency in the relationship of the position of the driver to the handedness of traffic. Most American cars produced before 1910 were RHD.[39] In 1908 Henry Ford standardised the Model T as LHD in RHT America,[39] arguing that with RHD and RHT, the passenger was obliged to "get out on the street side and walk around the car" and that with steering from the left, the driver "is able to see even the wheels of the other car and easily avoids danger."[73] By 1915 other manufacturers followed Ford's lead, due to the popularity of the Model T.[39]

In specialised cases, the driver will sit on the nearside, or kerbside. Examples include:

Where the driver needs a good view of the nearside, e.g. street sweepers, or vehicles driven along unstable road edges.[74]

Where it is more convenient for the driver to be on the nearside, e.g. delivery vehicles. The Grumman LLV postal delivery truck is widely used with RHD configurations in RHT North America. Some Unimogs are designed to switch between LHD and RHD to permit operators to work on the more convenient side of the truck.

Generally, the convention is to mount a motorcycle on the left,[75] and kickstands are usually on the left[76] which makes it more convenient to mount on the safer kerbside[76] as is the case in LHT. Some jurisdictions prohibit fitting a sidecar to a motorcycle's offside.[77][78]

Bird's-eye view of low beam light pattern for RH traffic, with long seeing range on the right and short cutoff on the left so oncoming drivers are not dazzled.

Most low-beam headlamps produce an asymmetrical light suitable for use on only one side of the road. Low beam headlamps in LHT jurisdictions throw most of their light forward-leftward; those for RHT throw most of their light forward-rightward, thus illuminating obstacles and road signs while minimising glare for oncoming traffic.

In Europe, headlamps approved for use on one side of the road must be adaptable to produce adequate illumination with controlled glare for temporarily driving on the other side of the road,[79]:p.13 ¶5.8. This may be achieved by affixing masking strips or prismatic lenses to a part of the lens or by moving all or part of the headlamp optic so all or part of the beam is shifted or the asymmetrical portion is occluded.[79]:p.13 ¶5.8.1 Some varieties of the projector-type headlamp can be fully adjusted to produce a proper LHT or RHT beam by shifting a lever or other movable element in or on the lamp assembly.[79]:p.12 ¶5.4 Some vehicles adjust the headlamps automatically when the car's GPS detects that the vehicle has moved from LHT to RHT and vice versa.[citation needed]

In the European Union, vehicles must be equipped with one or two red rear fog lamps. A single rear fog lamp must be located between the vehicle's longitudinal centreline and the outer extent of the driver's side of the vehicle.[80]

An Australian news source reports that some RHD cars imported to that country did not perform as well on crash tests as the LHD versions, although the cause is unknown, and may be due to differences in testing methodology.[81]

In most countries, rail traffic travels on the same side as road traffic. However, in many cases railways were built, often using LHT British technology, and road traffic switch the RHT while rail remained LHT. Examples include: Argentina, Belgium, Bolivia, Cambodia, Chile, Egypt, France, Iraq, Israel, Italy, Laos, Monaco, Myanmar, Nigeria, Peru, Portugal, Senegal, Slovenia, Sweden, Switzerland, Taiwan, Tunisia, Venezuela, and Yemen. In countries such as Indonesia it is the reverse (RHT for rails and LHT for roads). France is mainly LHT for trains, except for the classic lines in Alsace-Lorraine[82] which belonged to Germany when the railways were built before 1918. Metros and light rail sides of operation vary, and might not match railways or roads in their country. Trams generally operate at the same side as a road traffic due to a common sections with roads.

Of the 195 countries currently recognised by the United Nations, 141 use RHT and 54 use LHT on roads in general. A country and its territories and dependencies is counted once. Whichever directionality is listed first is the type that is used in general in the traffic category.

British colony until 1957. Ghana switched to RHT in 1974,[95][96] a Twi language slogan was "Nifa, Nifa Enan" or "Right, Right, Fourth".[97] Ghana has also banned RHD vehicles. Ghana prohibited new registrations of RHD vehicles after 1 August 1974, three days before the traffic change.

Greece

RHT

Grenada

LHT

British colony before 1974. Caribbean island.

Guatemala

RHT

Guinea

RHT

Guinea-Bissau

RHT

1928

Portuguese colony until 1974.

Guyana

LHT

British colony until 1970. One of the few countries in continental Americas are in LHT.

^Baedeker, Karl (1900). "Austria, including Hungary, Transylvania, Dalmatia and Bosnia". p. xiii–xiv. Retrieved 28 July 2017. In Styria, Upper and Lower Austria, Salzburg, Carniola, Croatia, and Hungary we keep to the left, and pass to the right in overtaking; in Carinthia, Tyrol, and the Austrian Littoral (Adriatic coast: Trieste, Gorizia and Gradisca, Istria and Dalmatia) we keep to the right and overtake to the left. Troops on the march always keep to the right side of the road, so in whatever part of the Empire you meet them, keep to the left.